Method for the hydrolytic precipitation of iron

ABSTRACT

The invention relates to a method for the hydrolytic precipitation of iron as jarosite from a sulphate-containing solution in connection with zinc recovery from zinc calcine. The recovey contains neutral leach, ferrite leach, zinc electrolysis and iron precipitation stages. The ferrite included in the calcine is leached with return acid of the electrolysis after which the iron present in ferrous form is neutralized and routed to an iron precipitation stage, where the iron is oxidised to the trivalent form using an oxygen-containing gas. Also present in the precipitation stage arc Na, K or NH 4  ions and jarosite nuclei.

The present invention relates to a method for the hydrolyticprecipitation of iron from a sulphate solution as jarosite. Asulphate-containing solution, with iron present in the solution indivalent form, is routed to an iron precipitation stage, where the ironis oxidised to the trivalent form using oxygen-containing gas. Alsopresent in the precipitation stage are Na, K or NH₄ ions and jarositenuclei.

Zinc calcine, obtained by roasting sulphidic zinc concentrates, isgenerally used as the starting material in the electrolytic preparationof zinc. The chief component of the calcine is zinc oxide, ZnO, but someof the zinc is also bound to iron in the form of zinc ferrite ZnOFe₂O₃.The amount of zinc ferrite is usually so considerable that zinc recoveryfrom that is unavoidable. Zinc oxide is easily soluble even at high pHvalues (3–5) whereas ferrite has to be leached at higher acid content.Ferrite leaching is performed in a separate stage, where both zinc andiron are dissolved according to the following reaction:ZnOFe₂O₃+4H₂SO₄==>ZnSO₄+Fe₂(SO₄)₃+4H₂O  (1)

The iron has to be precipitated from the solution obtained before thesolution can be returned to the neutral leach and from there to zincsulphate solution purification and electrolysis. There are no clearguidelines as to how much iron may be in the solution to be returned tothe neutral leach, but generally the level of 5 g/l Fe is consideredacceptable. The above process is described in e.g. U.S. Pat. Nos.3,434,947 and 3,493,365.

In industrial processes zinc oxide leaching, neutral leach, is generallycarried out in two stages at a pH of 2–5 and ferrite leaching can alsobe performed in two stages when the acid content is between 30–100 g/l.A precipitate is obtained from ferrite leaching, which contains thelead, silver and gold from the calcine. The recovery of these materialsmay be profitable in favourable conditions. The solution from ferriteleaching, which contains the dissolved zinc and iron, is very acidic,and if often pre-neutralised, before the iron is precipitated from it.Three iron precipitation processes are in use and in them the iron isprecipitated as either jarosite Na[Fe₃(SO₄)₂(OH)₆], goethite FeOOH orhematite Fe₂O₃.

When iron is precipitated as jarosite or goethite, a neutralising agentis to be used in precipitation to neutralise the sulphuric acid releasedin the reactions. Normally the neutralising agent is a calcine. Whenneutralisation is carried out with a calcine, the indium, gallium andmost of the germanium contained in the solution remain in the jarositeprecipitate in the same way as the zinc, copper and cadmium as well asthe indium, gallium, silver, gold and lead contained in the ferrite ofthe calcine. In most cases these valuable metals are lost in the ironprecipitate. In order to minimise the amount of calcine needed forneutralisation and therefore minimise losses as much as possible it isworth using pre-neutralisation.

When iron is precipitated as hematite, it occurs hydrolytically byoxidising from the solution without neutralisation, from which solutionthe iron is first reduced from trivalent to divalent form:2FeSO₄+O₂(g)+2H₂O==>Fe₂O₃+2H₂SO₄  (2)

The loss of valuable metals mentioned above is avoided in hematiteprecipitation. The precipitation of iron must however be performed in anautoclave at temperatures of about 200° C., which has essentiallyrestricted the adoption of the method, even though hematite is in factthe most environmentally friendly form of iron precipitate.

The hydrolytic precipitation of iron without neutralisation inatmospheric conditions would give great benefits, and a certain methodfor the precipitation of iron as jarosite is described in U.S. Pat. No.4,305,914. The method is based on the fact that jarosite is stable invery acidic solutions and that the partial precipitation of iron ispossible using the following balance reaction, when starting from aneutral ferri solution:3Fe₂(SO₄)₃+Na₂SO₄+12H₂O<=>2Na[Fe₃(SO₄)₂(OH)₆]+6H₂SO₄  (3)

After ferrite leaching, the solution is cooled and the residual acid isneutralised for instance with a calcine. After neutralisation, thesolution is heated and the iron may be precipitated from the solution inthe presence of sodium, potassium or ammonium ions and recycled jarositewithout the addition of a neutralising agent. The industrial realisationof this method has not however been successful, since the method is noteconomically profitable. In the first place, the solution containingtrivalent iron from the ferrite leaching must be cooled beforepre-neutralisation, so that the precipitation of iron does not takeplace at this stage. The second important factor is that iron cannot beprecipitated out in the precipitation stage at sufficiently lowcontents, because the precipitation rate slows down due to the largeamount of sulphuric acid generated in the reaction. In order for theprecipitation to be successful, the solution has to be diluted to abouthalf before precipitation. Precipitation of iron succeeds best from ahot solution, which means the solution has to be reheated almost to itsboiling point. Cooling and heating as well as dilution of the solutionmake it uneconomic.

The method now developed will eliminate the disadvantages of theprocesses described above and make it possible to precipitate ironhydrolytically from a sulphate solution as a very pure jarosite. Thesulphate solution, in which iron is dissolved in divalent ferrous form,is routed to the iron precipitation stage where the iron is oxidisedinto trivalent form using oxygen-containing gas. Present in theprecipitation stage are alkali ions such as sodium, potassium orammonium ions as well as jarosite nuclei, and the temperature of thesolution is at most that of the boiling point of the solution.Precipitation is thus carried out in atmospheric conditions. Theprecipitation method is suitable for processes where iron isprecipitated as jarosite. The essential features of the invention willbe made apparent in the attached claims.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1: the method of the invention is illustrated by flow chart 1.

DETAILED DESCRIPTION

It is possible with this method to treat for instance all the zincconcentrates on the market cost-effectively. Using this method it ispossible to recover all the valuable metals contained in zinc calcine inconditions that are technically easy to control. In the above-mentionedprocesses iron is always precipitated as jarosite from a trivalentsolution. The method of this invention is based on the fact that iron isprecipitated from a solution where the iron is in divalent ferrous form.When precipitation is performed from a ferrous iron solutionconsiderably greater precipitation rates are achieved than that in themethod described for example in U.S. Pat. No. 4,305,914. Iron can beprecipitated from a ferrous iron solution without a separate oxidationstage. The amount of zinc in the resulting jarosite is very small, only0.1–0.3%.

In flow chart 1 the method of the invention is combined with anelectrolytic zinc process, but please note that the method can beadapted for the precipitation of iron from other non-ferrous metalrecovery processes, such as those of copper and nickel. In the methodshown in flow chart 1 zinc calcine 1 is used as feed, which usuallycontains also other valuable metals. The zinc of the calcine is usuallyin the form of zinc oxide, but some of the zinc is also bound to theiron as zinc ferrite. The first treatment stage of the zinc calcine 1 isa neutral leach, which is preferable in two stages as is often thepractice. In the neutral leach stages the calcine is leached with adilute return acid solution of electrolysis so that the pH of thesolution is maintained in the range of 2–5. From the first neutral leachstage 2 the zinc sulphate solution 3 obtained is taken to electrolysisvia solution purification (not shown in detail in the diagram). Theprecipitate 4 from the first leach stage is taken to the second neutralleach stage 5, where the rest of the zinc oxide in the calcinedissolves.

The precipitate 6 from the second neutral leach stage 5 is routed toferrite leaching i.e. a strong acid leach 7, which is carried out usingreturn acid.

This stage may also be single or multi-stage. The H₂SO₄ content of thesolution in the strong acid leach is of the order of 30–100 g/l.Precipitate 8 is obtained from ferrite leaching, containing mainly lead,silver, gold and other insoluble compounds such as silicates and gypsum.The precipitate may be routed to a valuable metals recovery process.

The calcine iron in the solution 9 generated in ferrite leaching ismainly trivalent as normal, but the solution is not now taken to theusual pre-neutralisation and iron precipitation, instead in accordancewith the invention the iron is reduced to divalent form in a reductionstage 10. Reduction is carried out preferably using zinc concentrate orpossibly for instance with sulphur dioxide. The following reactionsoccur in reduction, depending on the reductant:Fe₂(SO₄)₃+ZnS

2FeSO₄+ZnSO₄+S^(o)  (4)Fe₂(SO₄)₃+SO₂+2H₂O

2FeSO₄+2H₂SO₄  (5)

The precipitate 11 generated from reduction stage 10 contains sulphurformed in reduction and possibly the concentrate routed surplus, and itmay be routed back to the roaster.

The reduction stage solution 12 is acidic, and has to be neutralisedbefore the iron is precipitated. The solution now contains divalent ironand there is no danger of precipitation even at high temperatures, sothat there is no need to cool the solution before pre-neutralisation.The solution can be neutralised as usual using zinc calcine, sinceferrous iron hydroxide is more soluble than zinc hydroxide, so that ironremains in solution.

Pre-neutralisation can be performed in many different stages of theprocess, but the most beneficial is in the second stage 5 of the neutralleach, where the iron (II)-bearing solution is neutralised at as high apH value as possible. In general the pH is raised at this stage to about3. When neutralising is performed in the second stage of the neutralleach, the neutraliser is the precipitate from the first stage i.e. theundissolved zinc calcine, which is fed at this stage together withferrite. The second neutral leach stage 5 solution 13 is routed to aniron precipitation stage 14. Iron is oxidised with oxygen-containing gasto trivalent in a solution that includes jarosite-forming ions (Na, K,NH₄ etc). Iron is then precipitated as jarosite according to thefollowing reaction:6FeSO₄+Na₂SO₄+1.5O₂+9H₂O

2Na[Fe₃(SO₄)₂(OH)₆]+3H₂SO₄  (6)

Since iron is not precipitated in the pre-neutralisation stage 5, aninternal circulation of iron in the strong acid leach and reductionstage is avoided, as only the ferrite precipitate that remainsundissolved in the neutral leach is taken to the strong acid leach stage7. The iron-containing solution 13 is routed after neutralisationdirectly to an iron precipitation 14. The iron precipitation stageyields a jarosite precipitate free of valuable metals and a zincsulphate solution 15, that has such a low amount of iron that thesolution can be taken to the first neutral leach stage.

It is known that the metals such as gallium, indium and germanium, whichare in zinc concentrate in small amounts dissolve during ferriteleaching and are always precipitated with ferric iron. The separation ofthese metals is very difficult if the iron is kept in ferric form thewhole time. As the iron in the solution going to pre-neutralisation isnow divalent, the recovery of the above-mentioned metals is possible forexample by neutralising some of the solution separately before it istaken to the actual neutralising stage 5. In this case the solution isneutralised preferably at least to a pH value of 4, whereby an iron-freeprecipitate containing Ga, In and Ge is achieved.

When using the method of this invention, it can be seen that thevaluable materials contained in zinc concentrate can be recovered wellat different stages and that the resulting jarosite is pure. When ironis precipitated from a ferrous iron solution, it is shown e.g. fromreaction (6), that only half the amount of sulphuric acid is generatedcompared with that generated in a ferric iron solution precipitation asin reaction (3). If zinc concentrate is used in the ferric ironreduction stage 10, the reduction reactions do not produce sulphuricacid, and thus only half as much sulphuric acid is generated as inconventional processes.

The flow chart shows a method where the solution coming from ferriteleaching is reduced in a separate reduction stage, but reduction canalso take place in connection with the strong acid leach stage without aseparate reduction stage.

The precipitation of divalent iron from a solution is described furtherby the following example.

EXAMPLE 1

A solution was treated that contained zinc sulphate corresponding to 100g/l Zn²⁺ and in addition 25 g/l ferrous iron, 2.5 g/l NH₄ and 10 g/l ofsulphuric acid plus an additional 200 g/l jarosite nuclei. The solutionwas heated to a temperature of 100° C. in a closed vessel. The slurrywas mixed well and O₂ gas was fed into it under the propeller, so thatthe partial pressure of the oxygen was held at 0.5 bar. The total ironand ferrous iron were monitored with samples, and the results are shownin the table below. The results also plainly show that in a few hoursthe iron can be made to precipitate to such a low level that it ispossible to return the solution to the first neutral leach stage. Basedon X-ray diffraction investigation the resulting precipitate wasjarosite. The filtering properties of the jarosite precipitate weregood. The amount of zinc left in the final precipitate was minimal.

This example indicates that sufficient iron is precipitated even thoughthe solution is only neutralised up to the point where it still contains10 g/l sulphuric acid, which corresponds to a pH value of about 1.Professionals in the field know that the results will improveconsiderably if the solution is neutralised further, for instance to apH value of 2–4, which is completely realistic. In addition the ammoniumcontent of the example was lower than is usually the case in zincprocesses. The required ammonium, NH₄, can also be fed as ammonia, NH₃,to the precipitation stage, wherein a little less acid is generated:6FeSO₄+2NH₃+1.5O₂+9H₂O

2NH₄[Fe₃(SO₄)₂(OH)₆]+2H₂SO₄  (7)

Instead of ammonium sodium hydroxide NaOH may also be used. Since suchunfavourable conditions also give such a good result, it is absolutelyclear that with higher pH levels, the results will be even better.

TABLE 1 Time Tot Fe Fe2 + NH4 H2SO4 Fe3 + h g/l g/l g/l g/l g/l 0 25.025.0 2.5 10.0 0.0 0.25 22.5 17.1 5.4 0.5 19.0 12.8 6.2 1 9.6 7.1 2.5 25.8 3.1 2.7 3 4.5 1.9 2.6 4 3.7 1.3 2.4 Final precip. Fe Zn S RDX: %34.2 0.26 13.8 Jarosite

1. A method for hydrolytic precipitation of iron as jarosite from asulphate-containing solution in connection with zinc recovery from zinccalcine, which recovery contains neutral leach, ferrite leach, zincelectrolysis and iron precipitation stages, comprising: leaching theferrite included in the calcine with return acid of the zincelectrolysis; after which reducing the ferric iron generated in theleach to ferrous form and neutralizing the ferrous iron; and in the ironprecipitation stage, oxidizing the neutralized ferrous iron intotrivalent form using oxygen-containing gas, with sodium, potassium orammonium ions and jarosite nuclei being present in the ironprecipitation stage and the temperature of the iron precipitation stagebeing at most that of the boiling point of the solution.
 2. A methodaccording to claim 1, wherein the ferric iron is reduced using zincconcentrate.
 3. A method according to claim 1, wherein the ferric ironis reduced using sulphur dioxide.
 4. A method according to claim 1,wherein the reduction of the ferric iron is carried out using zincconcentrate, which is added to the ferrite leach stage.
 5. A methodaccording to claim 1, wherein the ferrous iron contained in the solutionfrom the ferric iron reducing stage is neutralised with zinc calcine. 6.A method according to claim 1, wherein the ferrous iron contained in thesolution from the ferric iron reducing stage is neutralised by routingthe solution to the latter stage of a two-stage neutral leach of zinccalcine.
 7. A method according to claim 1, wherein the ferrous ironcontained in the solution from the ferric iron reducing stage isneutralised at least to a pH level of 4 in order to precipitate gallium,indium and germanium.
 8. A method for the hydrolytic precipitation ofiron as jarosite from a sulphate-containing solution in connection withzinc recovery, comprising: leaching zinc calcine in a two-stage neutralleach; electrolysing the zinc sulphate solution formed and leaching theremaining undissolved ferrite with return acid of die electrolysis in aferrite leach stage; reducing to divaleni form the ferrie iron dissolvedin the ferrite leach; after which routing the precipitate formed to azinc roaster and the solution to the second stage of the neutral leach;where the solution in the second stage of the neutral leach isneutralised using the residue from the first neutral leach stage;routing a residue from the second neutralisation stage to the ferriteleach and routing the neutralised solution to the iron precipitationstage; where the iron in the iron precipitation stage is precipitated asjarosite with oxygen in the presence of alkali or ammoniuni ions andjarosite nuclei.
 9. A method according to claim 8, wherein the ferriciron is reduced using zinc concentrate.
 10. A method according to claim8, wherein the ferric iron is reduced using sulphur dioxide.